In the past, sensor networks have been developed that can measure physical properties, such as pressure, and then communicate those measurements to locations where the measurements can be analyzed or stored. Most current sensor networks communicate using wires. The wires are used to supply power to the sensors and are also used for transmitting sensor signals. In general, a sensor receives energy through a wired electrical connection. The sensor uses the energy to produce a sensor measurement. The electronic circuit processing the signal from the sensor then encodes the sensor measurement into a sensor signal and transmits the sensor signal out through the wired electrical connection. A receiver obtains the sensor signal and recovers the sensor measurement.
A sensor measurement is not generally given directly as a real value of the measurand to be monitored and generally is without units. For example, a temperature sensor can return a sensor measurement of 5. The sensor measurement can be converted into a real temperature value by applying calibration information. Calibration information often takes the form of calibration coefficients. These coefficients can be obtained during a sensor calibration process. The calibration coefficients are used to build a bi-univocal mathematical correlation between the true physical value that is measured with a high accuracy by a reference sensor and the sensor's electrical response for that known value. In the example, the sensor measurement is 5 and the reference sensor produces a measurand of 50 degrees Celsius. Applying a multiplication calibration coefficient of 10 degrees Celsius to the value of 5 of the electrical response of our sensor can result in the true measurement of 50 degrees Celsius. This calibration coefficient of 10 degrees Celsius is then stored in the memory of the circuit for processing the signal from the sensor. Later, when the sensor produces a value of 5 due to the ambient temperature, the electronic circuit will multiply this value by the calibration coefficient and will indicate the real temperature of 50 Celsius degrees. In the general case, more complicated mathematical equations, with a large number of calibration coefficients are used for making the connection between the sensor measurement value and the real value of the measurand.
Some sensor networks use wireless sensors. A common approach is to use a battery to supply energy to the sensor. The sensor then wirelessly transmits the sensor signal to the receiver using electromagnetic waves. Battery powered wireless sensors are convenient because they do not require the costly and time consuming task of stringing wires. They do, however, require batteries. When batteries run out of energy, they must be replaced before the sensor can be used again.
Passive sensors are excited by an electromagnetic field. In other words, passive sensors obtain energy from an electromagnetic field. They typically have an antenna that converts the electromagnetic energy from an electromagnetic field into electrical energy to be applied to the sensor. The sensor then uses the energy to produce a sensor measurement and transmit a sensor signal.
FIG. 5, labeled as prior art, illustrates a sensor. The sensor has an active section 501 and a cover 503. An adhesive seal 502 is shown attaching the cover 503 to the active section 501. The adhesive seal 502 is one possible way to attachment method. For example, a quartz cover and a quartz active section as are desirable in many surface acoustic wave devices can be attached using glass frit technology or direct quartz to quartz bonding. A cover 503 is not required in all sensing applications but is often desirable in others. Minimizing stress when attaching a cover 503 to an active section 501 is necessary because stress on the active section 501 can negatively effect sensor operation.
Those skilled in the arts of radio communications, radio, or electromagnetic fields know of many different antenna configurations. These configurations range from simple dipole antennas to printed antennas, patch antennas, and spring antennas. Printed antennas are of particular current interest because they are printed or patterned directly onto a substrate, such as a printed circuit board, and are therefore extremely inexpensive to produce and integrate into an electronic system. Those skilled in the art of printed circuits, packaging, and system integration are aware of the numerous techniques for printing or patterning antennas and circuits onto substrates.
FIG. 6, labeled as prior art, illustrates a patch antenna. A substrate 607, such as a printed circuit board or kapton is patterned to have a patch 601 electrically connected to a first pad 603 by a wire 602. The other side of the substrate 607 has a ground plane 606 electrically connected to a second pad 605 by a second wire 604. In many applications, a through hole electrical connection will be used to make the electrical connection instead of the second wire 604.
FIG. 7, labeled as prior art, illustrates a spring antenna 701. A spring antenna 701 is simply a twisted or coiled piece of electrical conductor such as a wire.
Another passive wireless technology is radio frequency identification (RFID). An electromagnetic field excites an RFID module that contains identification information. Once excited, the chip transmits an identification signal containing the identification information. A typical use of an RFID module is to implant it in or attach it to cattle. The cattle are then tracked as they move through a detection area. The detection area has an electromagnetic field to excite the RFID module and a receiver to obtain the identification information.
In some RFID applications, the electromagnetic field contains an addressing signal. The addressing signal contains addressing information. The RFID module compares the addressing information to its own identification information. If the comparison reveals a match, then the RFID module transmits an identification signal. In this manner, a specific cow can be found in a herd. Such an application can be also seen at the monitoring and recognition of the trains that move through a certain fix position.
Matching networks can be used to couple signals between electronic components without losing too much energy. For example, the air through which an electromagnetic signal travels has a characteristic impedance. An antenna that receives the signal has a characteristic impedance. Furthermore, an electronic component has an input impedance and an output impedance. When a signal passes from one impedance to another, such as from an antenna to an electronic component, it loses energy. The energy loss can be minimized by matching the impedances. One way to match the impedances is to use an impedance matching network. Those skilled in the arts of electromagnetic fields or analog electronics know many impedance matching applications and solutions present in the real applications.
One use for a matching network is to match the impedances of an antenna and a surface acoustic wave (SAW) device. SAW devices are commonly used to filter signals or as sensors. Those skilled in art of SAW devices know of many varieties, applications, and uses of SAW devices.
Many sensors, particularly certain SAW sensors, require a cover that protects one side of the SAW sensor. The cover is often sealed against one side of the sensor. For many applications, a strain free seal is required because strain causes error in the sensor measurements.
Sensors, as discussed above, can operate passively to return a sensor measurement. However, passive sensors are rarely useful in applications requiring many sensors within a limited space. The reason is that the electromagnetic field stimulates all of the passive sensors and they all return sensor signals. The sensor signals interfere with one another and no signal is reliably read. Aspects of the embodiments directly address the shortcoming of current technology by producing an additional structure on a hybrid multifunctional system consisting of SAW sensor, printed antennas and other functional circuits without requiring additional processing steps.